Method of manufacturing a closed channel disk for a gas turbine engine

ABSTRACT

A disk having a closed channel construction and which is especially suited for supporting the rotating blades of a gas turbine engine is disclosed. In one embodiment, the disk supports the fan blades of a turbofan engine which extend radially therefrom. The disk structurally includes an upsteam web, a downstream web and a plurality of arcuate plugs disposed in end to end relationship between the webs to form a toroidal type structure having a substantially rectangular cross section. The disk is contoured to provide a level of torsional and bending stiffness which limits the most critical stage vibratory frequencies to values above the engine operating range while limiting the maximum value of the disk hoop stress during operation to a level below the durability limits of the disk material utilized. A method for making the disk is further disclosed.

This is a division of application Ser. No. 554,806, filed Mar. 3, 1975,now U.S. Pat. No. 3,970,412.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to gas turbine engines and more particularly toapparatus for supporting the fan blades of a turbofan engine.

2. Description of the Prior Art

The turbofan engine is the type of power plant most widely used on largeaircraft today. In the turbofan engine as distinguished from a turbojetengine, a portion of the working medium gases is pumped axially throughone or more compression stages and is exhausted directly to theatmosphere without passing through the core portion of the engine. Thecompression stages which exhaust directly to the atmosphere are calledfan stages and are generally positioned at the forward end of theengine. The ratio of the air flowing through the fan stages to airflowing through the core portion of the engine is referred to as thebypass ratio. The bypass ratio may be a different value for eachindividual engine model according to the performance requirements forthat power plant. In all turbofan engines, however, the fan stages makea substantial thrust contribution of between 30 and 75 percent to thetotal engine thrust at take-off with the actual contribution dependingprincipally upon the bypass ratio.

The size and weight of the fan stages varies proportionately with thebypass ratio. In one typical engine, the JT9D turbofan enginemanufactured by Pratt and Whitney Aircraft, a Division of UnitedAircraft Corporation, a single fan stage with large area flow pathaccommodates flow at a bypass ratio of approximately 5. Although theblades and the disk which comprise the fan stage of the JT9D engine arefabricated from titanium, the blades weigh collectively 450 pounds andthe disk weighs 470 pounds. Most turbofan engines in commercial servicetoday produce proportionately high fan thrust at takeoff and have fandisks which correspond in size to the JT9D disk. Such a massive disk isrequired to distribute the disk hoop stress which is generated as theblades and disk are rotated at speeds in excess of 3200 revolutions perminute during operation of the engine. Additionally, the disk is sizedto have adequate torsional and bending stiffness to resist deflectionunder the most critical vibratory excitations.

When compared to a turbojet engine operating at the same thrust level,one of the most attractive features of the turbofan engine is itsrelatively low level of noise generation. This low level of noisegeneration is made possible by reductions in the strength of the shearturbulence between the exhausted gases and the ambient medium. Toincrease the bypass ratio additional kinetic energy is removed from thecore gas stream to drive the fan stages. Resultantly, the exhaust gasvelocity of the core stream is diminished and a reduced level of shearturbulence obtains.

As engines having larger bypass ratios are designed and the size of thefan components is increased, significant performance increases can beachieved if the weight of the fan disk is reduced without sacrificingstructural support for the blades mounted thereon.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide support for theblades of a gas turbine engine.

In accordance with the present invention a disk which supports theblades in at least one stage of a gas turbine engine has a closedchannel construction; the closed channel disk is made by machining acircumferential channel into the outer periphery of a disk shaped memberto form an upstream and a downstream web, disposing a plurality ofarcuate plugs in end to end relationship between the upstream anddownstream webs at the periphery of the disk to form a toroidal typestructure having a substantially rectangular cross section, attachingthe arcuate plugs to the upstream and downstream webs, and machining aplurality of blade retention slots across the periphery of the diskthrough the upstream and downstream webs and through the arcuate plugsdisposed therebetween.

A principal feature in one embodiment of the present invention is thebond region between the webs and the arcuate plugs which is isolatedfrom the disk hoop stress by machining the blade retaining slots to adepth which is below the inner circumferential surfaces of the arcuateplugs. Another important feature is the ratio of the radial length ofthe webs to the axial distance between the upstream and downstream webswhich is varied to provide a level of torsional and bending stiffnesswhich limits the vibratory deflection of the blades during operation ofthe engine.

A principal advantage of the present invention is the substantiallyreduced weight of the disk made possible by the web-type construction.Concomitantly, the shaft thickness of a gas turbine engine incorporatingthe reduced weight disk can be correspondingly reduced while maintainingthe same critical rotor speed.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of the preferred embodiments thereof as illustratedin the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified side view of a turbofan engine having a partiallybroken away section which shows a portion of the fan assembly in crosssection;

FIG. 2 is a sectional view taken along the line 2--2 as shown in FIG. 1;and

FIG. 3 is a sectional view taken along the line 3--3 as shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A simplified side view of a turbofan engine 10 having a fan case 12which is partially broken away to reveal in cross section part of a fanassembly 14 is shown in FIG. 1. The fan assembly includes a plurality offan blades 16 which is attached to a fan disk 18. The inner portion ofthe disk is attached to a shaft 20 of an engine rotor assembly. As isshown in FIG. 2 each fan blade 16 has a root section 22 which engages acorresponding blade slot 24 in the disk 18. As is shown in FIG. 3, thefan disk 18 comprises an upstream web 26, a downstream web 28 and a baseportion 30. An arcuate plug 32 having an inner circumferential surface34 joins the upstream web to the downstream web at the periphery 36 ofthe disk.

The fan disk is a massive structure for supporting a multiplicity ofblades. In the JT9D turbofan engine referred to above 46 blades whichweigh approximately 10 pounds each are supported by the disk atrotational speeds in excess of 3200 revolutions per minute. As eachblade is thrust radially outward by centrifugal forces during operationof the engine, the blades collectively generate hoop stresses within thedisk. The maximum hoop stress at any one point is held to an acceptablevalue by providing an adequate material cross sectional area.

In addition to the material cross sectional area requirement, the diskmust have sufficient torsional and bending stiffness to limit the mostcritical stage vibratory frequencies to values above the operating rangeof the engine. Under critical vibratory conditions highly excitedsectors of the disk deflect in the upstream and downstream axialdirections to produce both bending and torsional stresses in the diskmaterial. Severe and often destructive bending stresses are produced inthe blades as the blades are forced to follow the distorted disk. Inconventional constructions, the axial thickness and cross sectional areaof a solid disk are increased to provide the required stiffness. Theincreased area, added for torsional and bending stiffness, exceeds thearea required to distribute the hoop stress and represents excessiveweight which is eliminated by more judicious use of the disk material inaccordance with the present invention.

In the embodiment shown in FIG. 3 a minimum cross sectional area isprovided which will reduce the maximum disk hoop stress to an acceptablevalue. The minimum disk area is then contoured to provide a structurehaving the desired torsional and bending stiffness. The box-typestructure including the upstream web 26 and the downstream web 28 asshown in FIG. 3 has the required stiffness. The upstream and downstreamwebs are spaced apart by an axial distance (A) and extend over a radiallength (B) as shown in FIG. 3. In the JT9D construction referred toabove, a ratio of the axial distance to the radial length (A/B) ofapproximately 0.7 is preferred and a ratio within the range 0.4 and 1.0is acceptable for most equivalent constructions. Although, it appearsthat a ratio greater than 1.0 would improve bending stiffness, the axialdistance (A) is limited by the width of the blade root which isindividually sized in each engine for optimum strength and weight of theblade.

Even through the solid fan disk of most former constructions isfabricated from titanium, the disk weight and, accordingly, the engineweight are substantially reduced by incorporating the closed channeldisk of the present invention. In the JT9D and comparably sized enginesthe disk weight is reduced by approximately 100 pounds. As a sidebenefit, the disk weight reduction is complemented by correspondingreductions in supporting components such as the rotor shaft which may bedecreased in material thickness while maintaining an equivalent criticalspeed.

The fan disk 18 is made by first machining a circumferential channelinto the outer periphery of a disk shaped member to form an elementhaving an upstream web 26 and a downstream web 28 at its periphery. Twoor more arcuate plugs 32 are inserted in end to end relationship intothe channel between the upstream and downstream webs at the periphery ofthe disk and are attached to the webs at that location to form atoroidal type structure having a substantially rectangular crosssection. In one embodiment the arcuate plugs are attached to theupstream and downstream webs by welding at the bond areas 38 as shown inFIG. 3, although diffusion bonding, brazing or other attaching methodsmay be effectively utilized. Although mechanical attaching means such asbolts or rivets generally add weight to the disk assembly, mechanicalmeans may be suitable for joining the arcuate plugs to the upstream anddownstream webs in some embodiments. The attached arcuate plugs increasethe stiffness of the disk and the resistance to axial deflection undervibratory conditions.

As is shown in FIG. 2, the blade slots 24 are machined into theperiphery 36 of the disk 18 to a depth which is radially inward of thecircumferential surface 34 of the plugs 32. Accordingly, the welds orbonds joining the plugs to the upstream and downstream webs of the diskare located beyond the live rim of the disk, in areas free fromcircumferential stress. Where two or more plugs 34 are utilized theopposing ends of adjacent segments terminate at one of the blade slots24.

Although the invention has been shown and described with respect topreferred embodiments thereof, it should be understood by those skilledin the art that various changes and omissions in the form and detailthereof may be made therein without departing from the spirit and thescope of the invention.

Having thus described a typical embodiment of my invention, that which Iclaim as new and desire to secure by Letters Patent of the United Statesis:
 1. A method for making a fan disk for a turbofan engine comprisingthe steps of:machining a circumferential channel into the outerperiphery of a disk shaped member to form an element having an upstreamweb and a downstream web; disposing a plurality of arcuate plugs, eachof which has an inner circumferential surface, into the channel in endto end relationship to form a toroidal type structure having asubstantially rectangular cross section; attaching the arcuate plugs tothe upstream and downstream web at the periphery of the disk; andmachining a plurality of slots across the periphery of the disk throughthe upstream and downstream webs and through one or more arcuate plugsdisposed therebetween, the most radially inward portion of each slotextending below the inner circumference of the plug.
 2. The inventionaccording to claim 1 wherein the slots extend across the periphery ofthe disk in an essentially axial direction.
 3. The invention accordingto claim 1 wherein the step of machining a plurality of slots includesthe further step of machining one of the slots at each juncture betweenadjacent arcuate plugs.